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The dielectric properties of pulsed laser deposited SrTiO3 thin films
Microelectronic Engineering 66 (2003) 891–895 www.elsevier.com / locate / mee
The dielectric properties of pulsed laser deposited SrTiO 3 thin films S.M. He*, D.H. Li, X.W. Deng, X.Z. Liu, Y. Zhang, Y.R. Li 1501 -1, College of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China, Chengdu, 610054, China
Abstract Dielectric thin films of SrTiO 3 (STO) were deposited on Y 1 Ba 2 Cu 3 O 7 2 x (YBCO) HTSC bottom layer by PLD method. X-ray diffraction results show that thin films were well crystallized and highly oriented in the (100) direction. Parallel capacitors were fabricated to investigate the low frequency dielectric properties of STO films. The current–voltage (I–V ) characteristics indicated that a Shottcky barrier exist at the interface between STO films and YBCO films. Dielectric dissipation of STO capacitor showed large frequency dispersion under high electric fields. It was explained by the space charge polarization near the interface. 2002 Elsevier Science B.V. All rights reserved. Keywords: STO; PLD; Dielectric; Thin films; Leakage current
1. Introduction There has been a significant interest in combining superconductors with ferroelectrics, such as thin films of SrTiO 3 (STO), in frequency or phase agile electronics, for example in tunable microwave filters and phase shifters [5]. In such devices, it is desirable to have a high dielectric tunability in a certain electric field range and low dielectric loss [1,2,4]. STO films have been deposited by sputtering, chemical vapor deposition, and pulsed laser deposition (PLD). PLD provides unique advantages for the deposition of multi-component oxide films because the stoichiometry of the target is easily reproduced in the films deposited.
2. Experimental STO / YBCO multilayers were deposited on a LaAlO 3 (100) substrate by the PLD method using a * Corresponding author. E-mail address: [email protected] (S.M. He). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)01017-1
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KrF excimer laser (248 nm wavelength). The targets were Y 1 Ba 2 Cu 3 O 7 2 x (YBCO) and SrTiO 3 ceramics. YBCO thin films were first deposited on LaAlO 3 (100) at substrate temperature of 780 8C in O 2 pressure of 20 Pa. Then SrTiO 3 thin films were deposited on the top of YBCO thin films at a substrate temperature of 680 8C in O 2 pressure of 5 Pa. The thickness of STO thin films was about 500 nm. After deposition, the films were annealed at 600 8C and oxygen pressure of 0.8 atm in the chamber for 30 min. The crystallinity of the STO / YBCO multilayers was examined by XRD. The morphology of the films was investigated using SEM and AFM. For measurements of low frequency dielectric properties of STO thin films, parallel capacitors were fabricated. The top electrode Au films was vacuum evaporated onto STO films through shadow masks. The area of the capacitor is 0.25 mm 2 . The capacitance and the dielectric dissipation were measured using an Agilent 4284A LRC meter with a signal level of 0.1 Vrms. The leak current was measured using an Agilent 34401A digital multimeter. In the measurement of C–V and I–V characteristics, the bias was applied to the Au top electrode, and the YBCO bottom electrode was connected to the ground.
3. Result and discussion The inductive measurements showed that the superconducting transition properties of YBCO thin films were excellent. Tc 0 590.6 K, DTc50.6 K. The surface of YBCO thin films characterized by SEM was smooth and homogeneous. Fig. 1 shows the XRD u 2 2u pattern of the STO / YBCO multilayers on LAO substrate. Only (l00) diffraction peaks of STO thin film and (00l) diffraction peaks of YBCO thin film exist in the XRD pattern. No grains with other orientation and impurity phases were detected. The full width at half maximum (FWHM) value of the rocking curve of STO(200) / YBCO(006) reflection was 0.738, as shown in Fig. 2. These indicated that BST / YBCO multilayers were highly oriented in the (00l) direction and STO and YBCO films of the perovskite structure were successfully obtained. Fig. 3 shows the AFM image of the STO films on YBCO bottom layer, the root mean square (RMS) roughness was 22 nm.
Fig. 1. XRD pattern of STO / YBCO multilayers on LAO sub. Only (l00) diffraction peaks of STO thin film and (00l) diffraction peaks of YBCO thin film exist.
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Fig. 2. Rocking curve of STO(200) / YBCO(006) peak, FWHM50.738.
The capacitance and the dielectric dissipation versus bias voltage measured at 77 K are shown in Figs. 4 and 5. When bias voltage increases from 0 to 5 V, the capacitance decreases from 5.8 (the corresponding effective permittivity was 1468) to 2.9 nF. The tunability was over 50%. The peaks of the C–V curves shift to 21.3 V. Although there are little differences between the C–V characteristics at different measuring frequencies, the dielectric dissipation of STO thin films showed large frequency dispersion especially under high bias fields. Dielectric dissipation rapidly increased with the bias voltage at 1 kHz while decreased a little at 100 kHz. This phenomenon could be attributed to the space charge polarization. Under the measurement frequency of 100 kHz, the maximum dielectric dissipation was 1.03% at zero bias. The space charges were mainly oxygen vacancies accumulated in the by-electrode Schottky depleted layer. STO thin films prepared by vacuum processes such as PLD usually contain oxygen vacancies. The oxygen vacancies form positive space charge under high electric fields by detrapping electrons. The STO films would have some characteristics of n-type semiconductors. The relationship
Fig. 3. AFM image of STO films on YBCO bottom layer (image size 232 mm). RMS522 nm.
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Fig. 4. Capacitance versus bias voltage of STO films at 77 K. The peak of the C–V curves shift to 21.3 V.
between the leak current and the bias is shown in Fig. 6. The bias was applied to the Au top electrode, YBCO bottom electrode was connected to the ground. It indicates that Schottky barriers exist at the interface between STO thin films and YBCO thin films. When the top electrode was positive biased, bias field added from the ‘body’ of the STO films to the interface between the STO films and the YBCO bottom electrode, which tend to increase the thickness of space charge layer near the bottom electrode, correspondingly increase the barrier height and restrict the leak current. The peaks of the C–V curves shift to 21.3 V, as shown in Fig. 4, could also be explained by the formation of series capacitance by STO film and the space charge layer near STO / YBCO interface [3]. However when reverse bias is applied over 2 V, the leak current rapidly increased. Under reverse bias, the Schottcky barriers do not contribute to DC resistance of the STO films. The large leak current could be
Fig. 5. Dielectric dissipation versus bias voltage of STO films at 77 K. The dielectric dissipation showed large frequency dispersion especially under high bias fields.
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Fig. 6. Leak current versus bias of STO films. The bias was applied to the Au top electrode. It indicates that Schottky barriers exist at the interface between STO films and YBCO films.
explained by detrapping of electrons from the oxygen vacancy and other defects under high electric fields hence decreasing the DC resistance.
4. Summary Au / STO / YBCO parallel capacitors were fabricated to study the low frequency dielectric property of STO thin films fabricated by the PLD method. Shottcky barriers were observed at the interface between the STO films and YBCO films. Capacitance changes of 50% were reached for the STO parallel capacitor. Dielectric dissipation of STO capacitor showed large frequency dispersion under high electric fields, which could be attributed to the space charge polarization. Under the measurement frequency of 100 kHz, the maximum dielectric dissipation was 1.03%.
References [1] [2] [3] [4] [5]
C. Ang, L.E. Cross, Z. Yu, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 78 (2001) 2754. C. Ang, Z. Yu, L.E. Cross, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 79 (2001) 818. K. Morito, T. Suzuki, S. Sekiguchi, H. Okushi, M. Fujimoto, Jpn. J. Appl. Phys. 39 (2000) 166. Y.-Ah Jeon, E.-S. Choi, T.-S. Seo, S.-G. Yoon, Appl. Phys. Lett. 79 (2001) 1012. O.G. Vendik, E.K. Hollmann, A.B. Kozyrev, A.M. Prudan, J. Superconductivity 12 (1999) 325.
The dielectric properties of pulsed laser deposited SrTiO 3 thin films S.M. He*, D.H. Li, X.W. Deng, X.Z. Liu, Y. Zhang, Y.R. Li 1501 -1, College of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China, Chengdu, 610054, China
Abstract Dielectric thin films of SrTiO 3 (STO) were deposited on Y 1 Ba 2 Cu 3 O 7 2 x (YBCO) HTSC bottom layer by PLD method. X-ray diffraction results show that thin films were well crystallized and highly oriented in the (100) direction. Parallel capacitors were fabricated to investigate the low frequency dielectric properties of STO films. The current–voltage (I–V ) characteristics indicated that a Shottcky barrier exist at the interface between STO films and YBCO films. Dielectric dissipation of STO capacitor showed large frequency dispersion under high electric fields. It was explained by the space charge polarization near the interface. 2002 Elsevier Science B.V. All rights reserved. Keywords: STO; PLD; Dielectric; Thin films; Leakage current
1. Introduction There has been a significant interest in combining superconductors with ferroelectrics, such as thin films of SrTiO 3 (STO), in frequency or phase agile electronics, for example in tunable microwave filters and phase shifters [5]. In such devices, it is desirable to have a high dielectric tunability in a certain electric field range and low dielectric loss [1,2,4]. STO films have been deposited by sputtering, chemical vapor deposition, and pulsed laser deposition (PLD). PLD provides unique advantages for the deposition of multi-component oxide films because the stoichiometry of the target is easily reproduced in the films deposited.
2. Experimental STO / YBCO multilayers were deposited on a LaAlO 3 (100) substrate by the PLD method using a * Corresponding author. E-mail address: [email protected] (S.M. He). 0167-9317 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-9317(02)01017-1
892
S.M. He et al. / Microelectronic Engineering 66 (2003) 891–895
KrF excimer laser (248 nm wavelength). The targets were Y 1 Ba 2 Cu 3 O 7 2 x (YBCO) and SrTiO 3 ceramics. YBCO thin films were first deposited on LaAlO 3 (100) at substrate temperature of 780 8C in O 2 pressure of 20 Pa. Then SrTiO 3 thin films were deposited on the top of YBCO thin films at a substrate temperature of 680 8C in O 2 pressure of 5 Pa. The thickness of STO thin films was about 500 nm. After deposition, the films were annealed at 600 8C and oxygen pressure of 0.8 atm in the chamber for 30 min. The crystallinity of the STO / YBCO multilayers was examined by XRD. The morphology of the films was investigated using SEM and AFM. For measurements of low frequency dielectric properties of STO thin films, parallel capacitors were fabricated. The top electrode Au films was vacuum evaporated onto STO films through shadow masks. The area of the capacitor is 0.25 mm 2 . The capacitance and the dielectric dissipation were measured using an Agilent 4284A LRC meter with a signal level of 0.1 Vrms. The leak current was measured using an Agilent 34401A digital multimeter. In the measurement of C–V and I–V characteristics, the bias was applied to the Au top electrode, and the YBCO bottom electrode was connected to the ground.
3. Result and discussion The inductive measurements showed that the superconducting transition properties of YBCO thin films were excellent. Tc 0 590.6 K, DTc50.6 K. The surface of YBCO thin films characterized by SEM was smooth and homogeneous. Fig. 1 shows the XRD u 2 2u pattern of the STO / YBCO multilayers on LAO substrate. Only (l00) diffraction peaks of STO thin film and (00l) diffraction peaks of YBCO thin film exist in the XRD pattern. No grains with other orientation and impurity phases were detected. The full width at half maximum (FWHM) value of the rocking curve of STO(200) / YBCO(006) reflection was 0.738, as shown in Fig. 2. These indicated that BST / YBCO multilayers were highly oriented in the (00l) direction and STO and YBCO films of the perovskite structure were successfully obtained. Fig. 3 shows the AFM image of the STO films on YBCO bottom layer, the root mean square (RMS) roughness was 22 nm.
Fig. 1. XRD pattern of STO / YBCO multilayers on LAO sub. Only (l00) diffraction peaks of STO thin film and (00l) diffraction peaks of YBCO thin film exist.
S.M. He et al. / Microelectronic Engineering 66 (2003) 891–895
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Fig. 2. Rocking curve of STO(200) / YBCO(006) peak, FWHM50.738.
The capacitance and the dielectric dissipation versus bias voltage measured at 77 K are shown in Figs. 4 and 5. When bias voltage increases from 0 to 5 V, the capacitance decreases from 5.8 (the corresponding effective permittivity was 1468) to 2.9 nF. The tunability was over 50%. The peaks of the C–V curves shift to 21.3 V. Although there are little differences between the C–V characteristics at different measuring frequencies, the dielectric dissipation of STO thin films showed large frequency dispersion especially under high bias fields. Dielectric dissipation rapidly increased with the bias voltage at 1 kHz while decreased a little at 100 kHz. This phenomenon could be attributed to the space charge polarization. Under the measurement frequency of 100 kHz, the maximum dielectric dissipation was 1.03% at zero bias. The space charges were mainly oxygen vacancies accumulated in the by-electrode Schottky depleted layer. STO thin films prepared by vacuum processes such as PLD usually contain oxygen vacancies. The oxygen vacancies form positive space charge under high electric fields by detrapping electrons. The STO films would have some characteristics of n-type semiconductors. The relationship
Fig. 3. AFM image of STO films on YBCO bottom layer (image size 232 mm). RMS522 nm.
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Fig. 4. Capacitance versus bias voltage of STO films at 77 K. The peak of the C–V curves shift to 21.3 V.
between the leak current and the bias is shown in Fig. 6. The bias was applied to the Au top electrode, YBCO bottom electrode was connected to the ground. It indicates that Schottky barriers exist at the interface between STO thin films and YBCO thin films. When the top electrode was positive biased, bias field added from the ‘body’ of the STO films to the interface between the STO films and the YBCO bottom electrode, which tend to increase the thickness of space charge layer near the bottom electrode, correspondingly increase the barrier height and restrict the leak current. The peaks of the C–V curves shift to 21.3 V, as shown in Fig. 4, could also be explained by the formation of series capacitance by STO film and the space charge layer near STO / YBCO interface [3]. However when reverse bias is applied over 2 V, the leak current rapidly increased. Under reverse bias, the Schottcky barriers do not contribute to DC resistance of the STO films. The large leak current could be
Fig. 5. Dielectric dissipation versus bias voltage of STO films at 77 K. The dielectric dissipation showed large frequency dispersion especially under high bias fields.
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Fig. 6. Leak current versus bias of STO films. The bias was applied to the Au top electrode. It indicates that Schottky barriers exist at the interface between STO films and YBCO films.
explained by detrapping of electrons from the oxygen vacancy and other defects under high electric fields hence decreasing the DC resistance.
4. Summary Au / STO / YBCO parallel capacitors were fabricated to study the low frequency dielectric property of STO thin films fabricated by the PLD method. Shottcky barriers were observed at the interface between the STO films and YBCO films. Capacitance changes of 50% were reached for the STO parallel capacitor. Dielectric dissipation of STO capacitor showed large frequency dispersion under high electric fields, which could be attributed to the space charge polarization. Under the measurement frequency of 100 kHz, the maximum dielectric dissipation was 1.03%.
References [1] [2] [3] [4] [5]
C. Ang, L.E. Cross, Z. Yu, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 78 (2001) 2754. C. Ang, Z. Yu, L.E. Cross, R. Guo, A.S. Bhalla, Appl. Phys. Lett. 79 (2001) 818. K. Morito, T. Suzuki, S. Sekiguchi, H. Okushi, M. Fujimoto, Jpn. J. Appl. Phys. 39 (2000) 166. Y.-Ah Jeon, E.-S. Choi, T.-S. Seo, S.-G. Yoon, Appl. Phys. Lett. 79 (2001) 1012. O.G. Vendik, E.K. Hollmann, A.B. Kozyrev, A.M. Prudan, J. Superconductivity 12 (1999) 325.